A gear puller is one option, but some cranks don't have clearance for the puller's arms to get behind the crank. Jacobs chuck remover wedges can also work. Slip a pair behind the crank, between the crank and the bottom bracket, tap them together and they can force the arm off just like removing the chuck from a drill press. Or, you can remove the retaining bolt and ride around for a while until the crank loosens on its own. This can happen surprisingly quickly. Best to just circle the block until this happens so you don't have too long a walk back to your home/shop.

Oh, and find a new bike shop for future work. But you knew that, I'm sure.